Presently, it is not uncommon for vehicles, such as trucks and buses, to be equipped with an exhaust restriction device. Such devices may be used for exhaust braking or for engine warm-up. Fundamentally, an exhaust restriction device need only comprise some means for restricting the flow of exhaust gas from an internal combustion engine. Restricting the exhaust gas increases the exhaust manifold pressure, i.e. "back pressure." The exhaust manifold pressure may be used to oppose the motion of the engine pistons for engine retarding or for warm up by increasing fueling rates and heat rejection. Thus, the engine and vehicle may be slowed and/or heated in relation to exhaust manifold pressure. Selective restriction of the flow of exhaust gas from the engine may therefore be used to selectively brake or warm up a vehicle.
Exhaust manifold pressure produced by an exhaust restriction device may be particularly useful in warming an engine during positive power operation. A cold engine may be more quickly warmed by placing the engine under load during positive power operation. Closing an exhaust restriction device during positive power creates an engine load because it makes it more difficult for the pistons to cycle in the cylinders. The exhaust restriction device creates this load by backing up warm exhaust gases in the engine and exhaust manifold which causes the engine to increase fuel consumption and increase heat rejection. Placing the engine under load increases the rate of raising vehicle cab temperature and decreases warm up time. Placing the engine under load by increasing exhaust manifold pressure is also desirable because it raises exhaust temperature, which promotes combustion and decreases carbon build up. Decreases in carbon help to alleviate emissions concerns, as well as problems with engine valve sticking.
One device for producing exhaust back pressure using a butterfly valve to restrict exhaust flow from a turbo charger outlet is disclosed in U.S. Pat. No. 5,079,921 to McCandless et al. In the device disclosed in this patent, the control of exhaust pressure results solely from opening and closing a butterfly valve adjacent to an engine turbocharger.
A device for producing a desired level of intake manifold pressure, as opposed to exhaust manifold pressure, is disclosed in U.S. Pat. No. 4,005,578 to McInerney. This device is also for use in conjunction with a turbocharger. The turbo compressor output is regulated by control of exhaust flow through the turbo turbine. This device does not control exhaust flow in response to the pressure in the exhaust system.
Devices for modulating exhaust flow are disclosed in U.S. Pat. No. 5,372,109 to Thompson et al. One of the disclosed devices includes a plunger to cover a bleed flow path. The plunger is controlled by computer controlled application of air or hydraulic fluid to the plunger. The plunger is not controlled by the application of exhaust gas to any actuation means. Another of the disclosed devices in Thompson includes a reed valve to cover a bleed flow path. The amount of deflection of the reed valve is the direct result of the application of exhaust pressure through the bleed flow path to the reed valve.
Some other exhaust restriction devices have been designed to provide a fixed maximum level of back pressure over a range of engine speeds. In such exhaust devices, control of the exhaust manifold pressure may be achieved by control of the restriction of exhaust gas flow by the device. These exhaust restriction devices may typically allow back pressure to build to a preset limit. Back pressure which exceeds the preset limit is relieved via a bypass around the closed exhaust restriction device. For example, U.S. Pat. No. 5,638,926 to McCrickard discloses an exhaust brake having a main tube and a bypass tube. During exhaust braking, the main tube is blocked with a rotatable valve. Back pressure is relieved by opening a bypass valve located at the downstream end of the bypass tube. Also see U.S. Pat. Nos. 4,750,459 and 4,682,674 to Schmidt, and U.S. Pat. No. 5,372,109 to Thompson et al., which disclose alternative bypass arrangements for an exhaust restriction device.
One impediment to the operation of known exhaust restriction devices is that they may expose the bypass valve, including its actuation means, to harsh temperatures and pollutants.
Bypass systems, preferably, should be constructed to remain operable under the harsh conditions experienced within an exhaust restriction device or removed from such harsh conditions. Exhaust gas typically contains carbon particles, water moisture, and other contaminants within it. Exposure of the moving parts of a bypass system to exhaust gas and its contaminants can cause the moving parts to corrode/oxidize and become coked/coated with carbon. Bypass valves, such as the one disclosed in the above-referenced McCrickard, Schmidt, and Thompson et al. patents, may become inoperable because of the build up of contaminants on the moving parts in the system. Accordingly, there is a need for an exhaust bypass system that is less prone to malfunction as a result of carbon, rust, or other contaminant build up on the moving parts of the bypass.
Furthermore, bypass systems should preferably be designed to avoid the exposure of heat sensitive elements of the bypass from being over exposed to high temperature exhaust gas. A bypass system may use a spring and/or electronic activators to open and close the bypass. These types of elements may not operate well under the fluctuating or extreme temperature conditions experienced within an exhaust restriction device. Accordingly, there is a need for an exhaust restriction device with a bypass actuator that is sufficiently thermally isolated and/or that has an acceptable tolerance of high temperature exhaust gas.
One of the designs described herein is a bolt-on bypass circuit which may be very effective at reducing the exposure of the bypass spring and/or electronic activators to exhaust gas temperatures. A bolt-on bypass may also add the benefit of flexible manufacturability which allows for a fixed flow area device or a variable area device with minimal manufacturing set up changes. A bolt-on bypass may be used with an exhaust restriction device that is pre-configured to accept the bypass. The exhaust restriction device may be provided originally with two or more plugged ports. The ports may be unplugged when a bolt-on bypass is added to provide exhaust gas flow to and from the bypass.
Another advantage of the exhaust restriction designs described herein, is the suitability of the designs to provide both an exhaust brake and a warm up device.